1. Field of the Invention
The invention in general relates to scan conversion apparatus, and particularly to a low cost system particularly well adapted for transmitting high quality pictures over low bandwidth transmission links.
2. Description of the Prior Art
A requirement exists for transmitting picture information over relatively narrow bandwidth channels, such as the conventional telephone line. Although facsimile machines are available for such transmission the quality and resolution of the received picture is not good enough for many intended purposes, for example in the remote diagnosis of transmitted X-ray pictures.
Systems for transmitting higher quality pictures includes the use of conventional TV cameras as well as slow scan cameras. Very considerable advantages can be realized by using a standard fast scan television camera instead of a slow scan camera in terms of relative cost for comparable picture qualities. In addition, in the slow scan camera arrangement a relatively long period of time is required to build up a displayed picture which makes initial adjustments and focusing, time consuming.
A system for transmitting high quality pictures over a narrow bandwidth transmission link, utilizing a standard TV camera is suggested in U.S. Pat. No. 3,792,194. Basically, each TV line of video from the standard camera is sampled many hundreds of times during the time of one TV line scan. The apparatus includes an analog-to-digital conversion, with the digital words describing each TV line being stored in a shift register and transmitted to the receiver via, for example, the telephone lines. At the receiver end, transmitted samples are stored in a register and provided to a rotating magnetic medium such as a video disc for display purposes. This system provides excellent quality transmitted pictures however where cost considerations are important, such technique requires the use of an extremely high speed analog-to-digital converter such converters being relatively expensive. In addition, the sampele and hold circuit required as well as the shift register at the transmitter location must be capable of operation of extremely high television data rates, a factor also adding to the cost of such system.
In another arrangement, as described in U.S. Pat. No. 3,564,127, only one picture element per TV line is sampled and then transmitted over a narrow bandwidth transmission link to a rotating magnetic medium such as a video disc. As the first picture element of each TV line is sampled and transmitted, the samples are placed into storage on the rotating disc (or drum) at evenly spaced apart locations. As the second picture element of each line is sampled and transmitted it is placed adjacent the first stored element on the disc, as the disc rotates, until such time as all of the picture elements have been sampled and reconstructed on the video disc. Such recording into adjacent locations during the revolution of the disc of an AM, or even a modulated signal, poses some technical problems with state-of-the art discs. In addition, if the frequency of picture element sampling is reduced, in order to be compatible with a further reduced bandwidth transmission link, picture elements transmitted adjacent in time will not appear adjacent on the final TV display, and due to the limitation of many narrow bandwidth transmission links, this operationwill result in objectionable ghost effects on the display.
A method to write slow scan signals on a video disc is described in U.S. Pat. No. 3,526,900. With this arrrangement, a slow scan TV line is sampled a number of times during the scan, the sampling being at a rate equivalent to a conventional TV line time. The sampled picture elements are distributed on the video disc as it rotates in order to build up a conventional TV signal. The TV raster is vertically scanned and sampled and rearranged so that transmitted samples are built up in a horizontal mode. Such technique described in the patent allows the recording of information onto the video disc, however, the sampled TV raster is produced by a slow scan camera and transmission at a conventional horizontal TV line rate would require the use of a bandwidth wider than conventional telephone bandwidths.